Nano Letters · 2020 · 21 citations · 30 references
The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr<sub>0.08</sub>(Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>1.92</sub>Te<sub>3</sub> to that of its nonmagnetic parent (Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>2</sub>Te<sub>3</sub>, to explore the cause. In (Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>2</sub>Te<sub>3</sub>, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr<sub>0.08</sub>(Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>1.92</sub>Te<sub>3</sub> with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb<sub>2</sub>Te<sub>3</sub>-based FMTI materials to very low temperatures.
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<i>Colloquium</i>: Topological insulators
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Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator
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